This invention relates to electrically conducting polymers, and is particularly directed to such polymers obtained from mono- or difunctional phenylacetylene-substitued Schiff's base monomers.
For many years synthetic organic polymers have attracted attention in a variety of electrical and electronic applications because of their outstanding insulator properties. However, since the discovery of the conducting properties of polyacetylene in the mid-seventies, replacement of metallic conductors with conductive polymers has been an important goal in chemically oriented research. During the past decade research efforts have intensified in obtaining improved, electroconductive, themosetting polymers useful, for example, in applications such as low-cost photovoltaic cells, moldable electrodes for use in light-weight batteries, composites, electromagnetic shielding devices, and the like.
The term "conductive polymer" is typicaly used to describe three distinct categories of polymeric materials. In the first category there are metal or graphite-filled polymers where conductivity is due solely to the filler. While most often these polymers exhibit high conductivity, a major drawback lies in the relatively large amount of filler which is needed, often changing the base polymer properties.
The second category includes "doped" polymer systems. These systems will typically consist of unsaturated polymers which contain no conductive filler but are treated to contain amounts of selected oxidizing or reducing agents. Although highly conductive polymers can be prepared by this means, most of the polymers will suffer from a loss of conductivity on simple exposure to normal atmospheric conditions or mild heat. Many of these polymers are difficult to prepare and isolate, and cannot be processed by ordinary polymer techniques.
The third category of conductive polymers includes polymers which are conductive in the pristine state. In this category, conductivity is due to the molecular configuration of the thermally post-cured polymer. Conductive polymers within this category are known in the prior art. See, for example, U.S. Pat. No. 4,178,430 to Bilow, and U.S. Pat. Nos. 4,283,557 and 4,336,362 to Walton. The monomeric precursors of these conductive polymers are ordinarily solids at room temperature. On heating, the monomers pass through a measurable temperature range in which they are in a viscous liquid or thermoplastic state. Within this range, prior to the onset of curing and the development of conductivity, these materials can be readily processed, in bulk, from the melt. The "processing window" of a conductive monomer herein is the temperature range between the endothermic minimum (where melting is just completed) and the temperature where polymerization just begins and is measured using a Differential Scanning Calorimeter (DSC) technique. This processing window where the monomer is in a liquid or thermoplastic state is a characterizing property of individual monomers and varies greatly with the monomer structure. In general, monomers having an unsymmetrical structure are likely to have a desirable wide processing window as compared to processing windows exhibited by symmetrical monomers.
Largely because of the noted deficiencies of the filled and doped polymers, interest in new intrinsically conductive or semi-conductive polymers remains high.